When the temp hits 100C and the thermo clutch on the fan tightens (or the electromagnetic fan clutch engages on some engines) you will hear "fan roar" at over 2-3000 RPM, which is normal.

Also, four-cylinder engines are noisier than sixes because their unbalanced second order vertical shaking force can produce a low frequency audible noise at medium to high revs.

If this noise is something new, I would verify proper operation of the fan and have the motor mounts inspected by a competent Mercedes tech.

Duke